Briefly, a fuel cell is a device wherein fuel (a reducing agent) and oxygen or air (an oxidizing agent) are continuously supplied to it from outside for electro-chemical reactions through which electric energy is taken out, and classified depending on its working temperature, the type of the fuel used, its applications, etc. Recently developed fuel cells are generally broken down into five types: a solid oxide type fuel cell, a molten carbonate type fuel cell, a phosphoric acid type fuel cell, a polymer electrolyte type fuel cell, and an alkaline aqueous solution type fuel cell, depending on kinds of using electrolytes.
These fuel cells use hydrogen gas resulting from methane or the like as fuel. More recently, a direct methanol type fuel cell (sometimes abbreviated as DMFC) relying on direct use as fuel of a methanol aqueous solution has been known in the art, too.
Among them, attention has now been directed to a solid polymer type fuel cell (hereinafter abbreviated as PEFC) having a structure wherein a solid polymer membrane is held between two kinds of electrodes and these components are further sandwiched between separators.
In general, this PEFC has a stacking structure wherein a plurality of unit cells, each having electrodes, are stacked on each side of the solid polymer membrane in such a way as to increase its electromotive force depending on what it is used for. A separator interposed between the unit cells is generally provided on its one side with a fuel gas feed groove for feeding fuel to one of the adjoining unit cells. With such a separator, fuel gas and oxidant gas are fed along its surfaces.
Known for the PEFC separator here are a graphite plate separator with a groove cut or otherwise formed in it, a separator obtained by molding of a carbon compound with carbon kneaded into a resin, a metal separator with a groove etched or otherwise formed in it, a metallic material separator with its surface portion covered up with a corrosion-resistant resin, and so on. If required, these separators are each provided with a fuel gas feed groove and/or an oxidant gas feed groove.
Besides such fuel cells of the stacking structure, for instance, there is a fuel cell for personal digital assistants, which requires not that large electromotive force, but must be of as thin a flat-type as possible. With that flat-type wherein a plurality of unit cells are arrayed in flat configuration and electrically connected together in series, however, there is a problem that the feed of fuel and oxygen becomes uneven from site to site.
As one possible approach to solving such uneven fuel feed problem, there has now been proposed a separator having a structure wherein a number of vertical through-holes are formed in a separator plane contiguous to a membrane-electrode assembly (MEA) to supply fuel and oxygen through them (JP(A)2003-203647).
In the present disclosure, the term “membrane-electrode assembly or MEA” is understood to refer to an assembly including electrode portions positioned between the fuel-feed-side separator and the oxygen-feed-side separator of a fuel cell, specifically, an assembly like a membrane wherein a collector layer, a fuel electrode, a polymer electrolyte, an oxygen electrode and a collector layer are stacked together in this order.
With that separator of such structure as explained above, however, there are problems such as difficulty with which wires are formed for making series connections between unit cells, complicated processes, and increased contact resistance due to wire connections.
With a prior art flat-type PEFC with the MEA put between the fuel-feed-side separator and the oxygen-feed-side separator as discussed above, there is another problem that as the MEA swells during power generation, it causes contact of the MEA with the fuel-feed-side separator or the oxygen-feed-side separator to become insufficient, resulting in an increased contact resistance. This problem could possibly be eliminated by using bolts to tighten together the fuel-feed-side separator and the oxygen-feed-side separator positioned on both sides of the MEA, thereby ensuring contact of the respective layers. However, provision of the width necessary for such tightening between the unit cells arrayed in flat configuration would arise yet another problem that the effective area of each unit cell diminishes.